1. Field of the Invention
The present disclosure relates to a mixer and a mixing method, especially to an active mixer and an active mixing method.
2. Description of Related Art
A mixer is operable to convert the frequency of an input signal, and the conversion is usually carried out through a switching circuit to multiply the input signal by a switching signal. The input signal to a mixer includes a radio-frequency signal and a local oscillation signal, while the output signal from the mixer is an intermediate-frequency signal which is generated by performing a frequency-shift operation to the radio-frequency signal in accordance with the local oscillation signal. Normally, a mixer is applicable to a frequency up-conversion circuit in a transmitting system or a frequency down-conversion circuit in a receiving system, and the mixer could be a passive mixer or an active mixer. Compared with an active mixer, a passive mixer has a better characteristic of linearity, but is unable to provide a conversion gain; therefore, as for a system adopting a passive mixer, the front-end circuit in the system has to provide a higher gain to suppress the noise generated by the rear-end circuit in the system; in addition, the isolation between the oscillation signal and the radio-frequency signal in a passive mixer is relatively worse, which possibly causes the problem of radiation of the oscillation signal. On the other hand, an active mixer is operable to convert a radio-frequency voltage into a current during its transistors operating in a saturation region, and carry out the frequency conversion with its switching circuit by outputting the current according to a switching signal; afterwards, the current outputted from the switching circuit is turned into an output voltage through an output load such that the active mixer is able to provide a conversion gain. Compared with a passive mixer, an active mixer provides a better effect on noise reduction due to its conversion gain; but the configuration of an active mixer has to distribute a voltage among a voltage-to-current converting circuit, a switching circuit and a load circuit under the supply limitations of the voltage, and reserve an appropriate margin of signal swing for each of the circuits for proper operation such that the design for an active mixer is relatively critical and the linearity is worse. More specifically, there must be some give and take in the design of an active mixer. Take a double-balanced active mixer (a.k.a. Gilbert Cell) for example; although it is able to reduce flicker noise by reducing the current flowing through a transistor (hereafter, switching transistor) for receiving a local oscillation signal, this manner leads to the increase of the effective impedance seen by another transistor (hereafter, input transistor) for receiving a radio-frequency signal, and, consequently, the input transistor is likely to enter a triode region from a saturation region when receiving the radio-frequency signal of a higher amplitude and then degrade the linearity of the active mixer. In addition, although it is possible to raise the gain by increasing the load resistance at the output end for the intermediate-frequency signal of the active mixer, this manner similarly decreases the current flowing through the switching transistor and degrades the linearity of the active mixer. In addition to the above-mentioned problems, as the development of semiconductor processes improves, the maximum operation voltage available to a miniaturized transistor is gradually decreased. Accordingly, if the first concern is to make sure that the switching and input transistors operate in the saturation region to keep the linearity (which means that the linearity concern dominates), the margin of signal swing at the output end of the intermediate-frequency signal is sacrificed (which means that the conversion gain is sacrificed), which is unfavorable to the following signal processing procedure; but if the first concern is to ensure the margin of signal swing at the output end, the probability of the switching and input transistors entering the triode region from the saturation region increase, which leads to the loss in the linearity of the active mixer. In order to solve the problems due to the lower and lower operation voltage, some prior arts make use of source-degeneration techniques for the improvement; however, the source de-generation techniques leads to the reduction in gain and the increase in circuit area, which also leaves a designer the dilemma.
People who are interested in the prior arts may refer to the following documents: US patent application of publication number 20090029668; and U.S. Pat. Nos. 5,548,840, 6,078,802, 6,639,447, 7,816,970, 7,948,294.